1. Field of the Invention
The present invention relates to an image forming device, such as a laser printer. More particularly, the invention relates to a bias applying power source for applying a bias (dc voltage) to a transfer roller.
2. Description of the Prior Art
Ordinarily, image forming devices such as laser printers include a photosensitive drum and, provided around this photosensitive drum in the direction of the drum's rotation, a charging device, a scanning device, a developing roller, and a transfer roller.
As the photosensitive drum rotates, the charging device applies a uniform charge to the surface of the photosensitive drum. Next, the surface of the photosensitive drum is exposed to the high-speed scanning of a laser beam emitted from the scanning device to form a latent image based on image data. When the surface of the photosensitive drum is rotated into the developing roller, toner carried on the surface of the developing roller is supplied to the latent image formed on the photosensitive drum and is selectively carried thereon, forming a toner image (visible image). Subsequently, the toner image carried on the surface of the photosensitive drum is rotated in opposition to the transfer roller and is transferred to a sheet of paper passing between the photosensitive drum and the transfer roller by a transfer bias applied to the transfer roller.
Cleaning devices well known in the art are provided in these types of image forming devices to clean toner deposited on the transfer roller before or after the image forming operation or between the feeding of each sheet of paper. This type of image forming device includes a power source for applying a transfer bias to the transfer roller. The power source includes a forward bias applying circuit and a reverse bias applying circuit that are connected to the transfer roller in series. The forward bias applying circuit applies a forward transfer bias that is lower than the surface potential of the photosensitive drum contacting the transfer roller. The reverse bias applying circuit applies a reverse transfer bias that is higher than the surface potential of the photosensitive drum.
During a transfer operation, the forward bias applying circuit applies a forward transfer bias to the transfer roller to transfer the toner image onto the paper. During a cleaning operation, the reverse bias applying circuit applies a reverse transfer bias to the transfer roller in order to electrically expel toner deposited on the transfer roller onto the surface of the photosensitive drum.
In these types of transfer bias applying power sources, the forward bias applying circuit is usually controlled by a constant current, in order that a constant transfer current can be applied at all times, even when the resistance on the transfer roller end varies due to ambient changes, for example (here and hereafter the resistance value includes the photosensitive drum and the paper). In this type of constant current control, the transfer current value is determined by detecting the output voltage of the forward bias applying circuit with a detection circuit provided therein and calculating the resistance on the transfer roller end based on this output voltage.
However, the forward and reverse bias applying circuits are connected to the transfer roller in series in this type of power source (in other words, the transfer roller has only one output terminal). Accordingly, when calculating the resistance on the transfer roller based simply on the output voltage from the forward bias applying circuit detected by the detection circuit, the calculated resistance contains the resistance in the reverse bias applying circuit as an error, preventing an accurate calculation of the resistance value on the transfer roller end.